Sclereid presence and distribution were studied in leaves from 50 species of Dionysia (Primulaceae). Of 16 species not previously investigated, 11 were shown to possess sclereids. The sclereids are dermal, terminal or diffused (according to the position in the leaf) and develop from different tissues, and hence are not homologous structures. The presence of different types of sclereids in different species is discussed. Sclereid characters are optimized on a cladogram based on an analysis of three DNA regions. Some clades are associated with certain types of sclereids. Terminal sclereids are most common in Iran, and diffused sclereids in Afghanistan. The evolution of sclereids in the genus is discussed, and a redefinition of what can be called a sclereid in Dionysia is presented.